This invention relates to a system and method for aligning a scanning surface of an inspection probe relative to a workpiece surface being inspected by the probe. More particularly, the present invention relates to a system employing eddy current elements for providing substantial parallel alignment between the scanning surface of the inspection probe and the workpiece surface under inspection.
In industrial non-destructive inspection operations which typically employ inspection probes, such as for example eddy current or ultrasonic probes or the like, for detection of flaws in a workpiece under inspection, it is highly desirable to use techniques which can effectively reduce the time and cost required to inspect a workpiece without compromising the quality of the inspection operation. Implementing such effective techniques becomes particularly difficult when the workpiece to be inspected has a large surface area (e.g., the skin of an aircraft wing) which must be thoroughly inspected not only to detect microscopic flaws therein but also to pinpoint the location of the detected flaws.
To address the need for effective inspection techniques, inspection probes have been proposed which provide an increased scanning surface area so as to speed up the inspection operation. A typical approach employed in the fabrication of these proposed inspection probes is to spatially correlate a plurality of elementary inspection elements over the scanning surface of the probe. This approach can provide an array of inspection elements which not only has an increased scanning surface area but also has the resolution and sensitivity required to accurately and precisely detect and locate microscopic flaws in the workpiece under inspection.
For instance, probes used in eddy current inspection may thus comprise a plurality of inspection eddy current elements arrayed throughout the scanning surface of the probe. Although impressive improvements have been made with the use of such proposed eddy current probes, significant problems remain. For example, undesirable electrical signals can arise during routine eddy current inspections due to variations in the spacing between each inspection eddy current element and the underlying surface of the workpiece undergoing inspection. This phenomenon is generally referred to as the lift-off effect, a term of art used to characterize the undesirable signals which result from the spacing variations. The lift-off effect is capable of corrupting actual eddy current measurement signals to the point of dangerously concealing the presence of actual flaws in the workpiece under inspection. Accordingly, the lift-off effect can significantly impair eddy current inspection operations and can jeopardize the reliable use of such proposed eddy current probes (e.g., probes using an array of inspection eddy current elements) since their ability to reliably and accurately detect flaws in the workpiece is diminished. Similar undesirable effects, due to spacing variations between the scanning surface and the workpiece surface, can also affect other kinds of inspection probes, such as for example ultrasonic inspection probes. Moreover, inspection of workpieces having a substantially curved surface, for example, remains especially troublesome due to the lift-off effect since it is particularly difficult to maintain a uniform relationship between the inspection elements arrayed over the scanning surface of the probe and the workpiece surface.
It is therefore desirable to provide a system and method for aligning the scanning surface of the probe so as to maintain uniform spacing between each inspection element therein and the surface under inspection. Maintaining such uniform spacing therebetween would eliminate erroneous indications resulting from variations in the spacing between each of the individual inspection elements and the workpiece being inspected.
A known typical system employed to align the scanning surface of inspection probes may use a controller which uses a priori information about the shape of a generic workpiece surface. For instance, a scan plan of the generic workpiece surface can be programmed off-line and stored in the controller prior to starting the inspection operation. This known system, however, has several disadvantages since the a priori information stored in the controller generally cannot take into account dimensional tolerances between the numerous workpieces being inspected in a production environment, and therefore this system is usually unable to provide the continuous alignment required to substantially reduce erroneous indications due to the lift-off effect. Moreover, this known system in general requires additional off-line programming to align the inspection probe over workpieces having a geometry different from the chosen generic workpiece.